In recent years, solid-state imaging elements such as a CCD image sensor and a CMOS image sensor are mounted in imaging devices such as a digital video camera and a digital still camera, and a variety of electronic equipment provided with an imaging function, such as a scanner, a facsimile and a cell phone with a camera. For example, the solid-state imaging element stores, in a charge storage region forming part of the photodiode, electric charges generated by photoelectric conversion of a photodiode formed in a substrate, and also generates a signal constituting image data based on the electric charges read from the charge storage region into a read region at predetermined timing.
Such a solid-state imaging element has a problem of a dark current and a white spot which occur due to electric charges generated not by incidence with light (a phenomenon of a white signal being generated by a large amount of dark current). A dark current and a white spot occur mainly due to supply of electric charges to the charge storage region from interface states on a top surface of the substrate (a surface on a side where a variety of regions such as the charge storage region and a read region are formed, and the same shall apply hereinafter). Therefore, a shield region made up of impurities of a different conductive type from that for the charge storage region has been formed between the top surface of the substrate and the charge storage region, thereby suppressing supply of the electric charges from the interface states on the top surface of the substrate to the charge storage region, to suppress occurrence of a dark current and a white spot.
However, when the shield region as thus described is formed in the substrate, the electric charges pass through the shield region at the time of reading the electric charges from the charge storage region into the read region. At this time, a potential in the shield region becomes significantly large as compared to a potential in the charge storage region, thus leading to deterioration in read efficiency of the electric charges from the charge storage region into the read region. When a large amount of electric charges cannot be read and left in the charge storage region, those electric charges are mixed with electric charges that are stored in the next photoelectric conversion to cause occurrence of a residual image in image data, which is problematic.
Therefore, Patent Document 1 proposes a solid-state imaging element where a channel region is formed immediately below a gate electrode, and part of a charge storage region extends to a place immediately below the gate electrode and is in contact with the channel region. This solid-state imaging element will be described hereinafter with reference to a drawing. It is to be noted that the solid-state imaging element to be described hereinafter is one where an n-type charge storage region is provided in a p-type substrate, and the charge storage region stores electrons.
FIGS. 10A to 10C are diagrams showing a structure and potentials of a conventional solid-state imaging element. FIG. 10A is a diagram showing a cross section of the solid-state imaging element, and a broken arrow in the drawing indicates an electron channel (read channel) at the time of reading electrons stored in a charge storage region 104 into a read region 105 (at the time of read). FIG. 10B shows a potential on the read channel at the time of storing the electrons in the charge storage region 104 (at the time of storage). FIG. 10C shows a potential on the read channel at the time of read.
As shown in FIG. 10A, a solid-state imaging element 100 is provided with: a p-type substrate 101; a gate insulating film 102 formed on a top surface 101a of the substrate 101; a gate electrode 103 formed on the gate insulating film 102; the n-type charge storage region 104 formed at a position inside the substrate 101 and apart from the top surface 101a of the substrate 101; the n-type read region 105 formed at a position inside the substrate 101 and on the opposite side to the charge storage region 104 with the gate electrode 103 interposed therebetween; a p-type first channel region 106 and a p-type second channel region 107 formed at a position inside the substrate 101 and immediately below the gate electrode 103; a p-type shield region 108 formed at a position inside the substrate 101 and between the top surface 101a of the substrate 101 and the charge storage region 104; and an element separation part 109 formed by STI (Shallow Trench Isolation) or the like around an element region of the substrate 101 which is formed with each of the above parts 102 to 108.
Further, in this solid-state imaging element 100, part of the charge storage region 104 is extended to a place immediately below the gate electrode 103 to come into contact with the lower end of the first channel region 106. Moreover, a concentration of p-type impurities in the first channel region 106 is made lower than a concentration of the p-type impurities in the shield region 108, and made higher than a concentration of the p-type impurities in the second channel region 107.
In this solid-state imaging element 100, an electron read channel is one from the charge storage region 104 to the read region 105 through the first channel region 106 and the second channel region 107. Since this reduces a difference between a potential in the charge storage region 104 and a potential on the read channel, it becomes possible to improve the efficiency in reading the electrons from the charge storage region 104 into the read region 105 at the time of read shown in FIG. 10C. Further, since a downward inclination of the potential from the first channel region 106 to the second channel region 107 is formed, at the time of storage shown in FIG. 10B, electric charges supplied from interface states immediately below the gate electrode 103 can be efficiently discharged to the read region 105 where the potential has been reset to a predetermined potential.